Effects of Aging and Spectral Shaping on the Sub-cortical (Brainstem) Differentiation of Contrastive Stop Consonants
DEDICATION With love and appreciation to my family, friends and teachers. iv ACKNOWLEDGEMENTS It is with sincere gratitude that I acknowledge the support and help of my advisor Ashley W. Harkrider who played a major role in writing this dissertation, in my Ph.D. study and research, and in my career. Besides my advisor, I would like to express gratitude to my professor Mark S. Hedrick for his valuable guidance and contribution to stimuli preparation, the set-up and design of the behavioral
... he behavioral experiment, and the analysis of perceptual data of this dissertation. Many thanks also go to my committee members, Professors Patrick N. Plyler and Gregory C. Petty for their insightful comments that improved the presentation and contents of this dissertation. v ABSTRACT Purpose: The objectives of this dissertation are to: (1) evaluate the influence of aging on the sub-cortical (brainstem) differentiation of voiced stop consonants (i.e. /b-dg/); (2) determine whether potential aging deficits at the brainstem level influence behavioral identification of the /b-d-g/ stimuli, (3) investigate whether spectral shaping diminishes any aging impairments at the brainstem level; and (4) if so, whether minimizing these deficits improves the behavioral identification of the speech stimuli. Subjects: Behavioral and electrophysiological responses were collected from 11 older adults (> 50 years old) with near-normal to normal hearing and were compared to those of 16 normal-hearing younger adults (control group). Stimuli and Methods: Speech-evoked auditory brainstem responses (Speech-ABRs) were recorded for three 100-ms long /b-d-g/ consonant-vowel exemplars in unshaped and shaped conditions, for a total of six stimuli. Frequency-dependent spectralshaping enhanced the second formant (F2) transition relative to the rest of the stimulus, such that it reduced gain for low frequencies; and increased gain for mid and high frequencies, the frequency region of the F2 transition in the /b-d-g/ syllables. Behavioral identification of 15-step perceptual unshaped and shaped /b-d-g/ continua was assessed by generating psychometric functions in order to quantify stimuli perception. Speech ABR peak amplitudes and latencies and stop consonant differentiation scores were measured for 6 stimuli (3 unshaped stimuli and 3 shaped stimuli). Summary of Findings: Older adults exhibited more robust categorical perception, and subtle sub-cortical deficits when compared to younger adults. Individual data showed fewer expected latency patterns for the /b-d-g/ speech-ABRs in older adults as opposed to younger adults, especially for major peaks. Spectral shaping improved the stop consonant differentiation score for major peaks in older adults, such that it moved older adults in the direction of the younger adults' responses. Conclusion: Sub-cortical impairments at least those measured in this study do not seem to influence the behavioral differentiation of stop consonants in older adults. On the other hand, cue enhancement by spectral shaping seems to overcome some of the deficits noted at the electrophysiological level. However, due to a possible ceiling effect, improvements to the originally robust perception of older adults, at the behavioral level were not found. Significance: Aging seems to reduce the sub-cortical responsiveness to dynamic spectral cues without distorting the spectral coding as evident by the "reparable" agerelated changes seen at the electrophysiological level. Cue enhancement appears to increase the neural responsiveness of aged but intact neurons, yielding a better subcortical differentiation of stop consonants.